RS-485 is a two-wire connection, with the pair of wires transmitting and receiving data. With the RS-485 feature enabled (ATIO 2 command), the 9-pin serial I/O connector on the front of the Raveon data radio modem model M5 and M7 will be put into the RS-485 mode of operation. The serial I/O connector is a female 9-pin D-subminiature connector having the following pin configuration.

Front-view of DB-9 connector on M7 data radio modem (female)

The following table lists the pin functions for the input and output pins on the DB9 connector when it is in the RS-485 mode.

Pin #

Name

Dir

Function

Level / Specification

1

–

Do not connect

Do not connect this pin to anything.

2

B (-)

I/O

B

Inverting RS-485 data in line

3

Out –

out

Z

Connect to pin 2

4

A (+)

I/O

A

Non-inverting RS-485 data out line

5

GND

Ground

Connect to earth ground.

6

–

Do not connect

Do not connect this pin to anything.

7

Out +

out

Y

Connect to pin 4

8

9

Vin

In/Out

DC Power

DC Power in or out if unit is powered using DC in jack.

The RS485 differential data line consists of two pins, A and B.

B TxD-/RxD- aka inverting pin which is negative (compared to B) when the line is idle.A TxD+/RxD+ aka non-inverting pin which is positive (compared to A) when the line is idle.

Configuring The M7 Radio Modem

To configure the M7 modem for RS-485 operation, use the ATIO x command. ATIO 2 sets the serial port for standard RS-485, full duplex. ATIO 3 sets the serial port for RS-485 simplex mode. In most cases, RS-485 simplex is preferred (ATIO 3). Also, turn off character echo using the ATE0 command. If echo is on, communications will be garbled when the radio is in the command mode.

In simplex RS-485 mode, the M7’s serial data receiver is internally disabled whenever it sends a character out the serial port, so that it will not receive its own data. For most application use the RS485 simplex mode, ATIO 3. If you are using two M7 modems with RS485 on both units, you must use the simplex RS485 mode instead of the duplex RS485 mode, or the modems will enter an infinite loop-back condition. If you wish to run the RS485 in full-duplex mode, the interface must be wired with two separate pairs of wires; A&B on one pair, and Out+&Out- on the other pair.

Raveon’s RS-485 I/O circuit uses 3.3V logic to drive the lines, and the RS485 pins are ESD protected to ±15kV and 7kV human body model. Input current is less than 150uA. Output current when not driving the line is less than 50uA.

Bias and Termination Resistors

RS-485 installations typically have a termination resistor across the A and B lines. For low-speed operation (<57600 baud), this resistor is probably not necessary. If you wish to use a terminating resistor, a value of 150 ohms should work in most applications.

These A/B pin names are all in use on various types of equipment. The RS485 signaling specification states that signal A is the inverting or ‘-‘ pin and signal B is the non-inverting or ‘+’ pin. The same naming is specified in the NMEA standards.

When an RS-485 network is in an idle state, all nodes are in listen (receive) mode. Under this condition there are no active drivers on the network. All drivers are tri-stated. Without anything driving the network, the state of the A and B line is unknown. If the voltage level at the receiver’s A and B inputs is less than ±200mV the logic level at the output of the receivers will be the value of the last bit received. In order to maintain the proper idle voltage state, bias resistors must be applied to force the data lines to the idle condition.

Terminal Blocks

To make it easy to connect your RS-485 device in the field to a Raveon data radio modem, we off a simple screw-down terminal block. It has a male DB9 connector on it, so it plugs directly into the M7 radio modem.

The field termination terminal block connector show above is Raveon part number DB9M-TB. Contact sales@raveon.com to order this part. Customers purchasing the M7 radio modem with the RS-485 option should also order this field termination block because it makes installation very easy.

Radio Manager

If you are going to use Raveon’s RadioManager software to configure your product or communicate with it, the RS-488 interface must be configured as RS-485 SIMPLEX. Duplex RS-485 will not work with RadioManger. When you configure your data radio modem to operate in RS-485 mode, you should also turn off “character echo”. The ATE0 command disables charactor echo in the command mode. Echo must be off for RadioManger to talk to a modem using RS-485.

RadioManager versions newer than 5.4 will automatically turn off character echo if they detect that the connected radio is using RS-485 to communicate with RadioManger.

TDMA systems are often used to send consistently timed, periodic data. In these situations, system designers face the challenge of adjusting the rate data is sent to the modem to match the TDMA parameters. With Automatic Message Delineation, this constraint is removed. Instead, the modem is allowed to detect message boundaries and send only the most recent data message when TDMA airtime is available.

Under standard operation, TDMA modems buffer all data received and transmit as much as possible when time slots are available. With Automatic Message Delineation, only a single message will be buffered at a time and the most recent message will be sent over-the-air when a time slot is available.

Messages are detected using inter-character timeout. Characters that are received in rapid succession are grouped into a single message, while longer spaces between characters create an automatic message boundary. Upon reception of a new message, previously received data is cleared and the new message is queued for transmission. Message data is also cleared upon transmission.

Automatic Message Delineation automatically creates new messages and only sends the most recent data.

To enable Automatic Message Delineation, issue the command MSGDEL 1 to the modem. To control inter-character timing, the ATR3 setting may be used. The default inter-character timeout of 20 milliseconds is usually sufficient for most systems. Automatic Message Delineation is only in effect when TDMA is enabled.

An upgrade detection engine has been added to the radio. Upgrades to D70 or later will process upgrade steps automatically, keeping functionality as identical as possible. Operation should always be verified after upgrade.

The CONFIG command is now supported as a user-accessible command to retrieve all radio configuration

The M7-GX series of GPS transponders may be directly connected to any Raymarine A, C, E or G series display. All of these Raymarine displays have at least one NMEA-0183 port. This allows them to be used with Raveon’s RavTrack series of GPS radio transponders to make a complete GPS tracking system. For this article, the Raymarine E80 product will be used as an example but the other models in the Raymarine line have similar integration steps.

When connected to the M7-GX GPS radio transponder, the E80 map will show the location of the user plus the location of all other transponders within radio range. This unique feature allows a user to quickly, easily and inexpensively make a portable AVL system for tracking cars, trucks, race cars, construction equipment or anything Raveon’s M7-GX transponder may be installed on.

The Raymarine displays have a built-in interface for NMEA 0183 devices, a serial data port intended for RS422/RS232 operation. The Raymarine E80 supports RS232 levels on the communications lines as well as the RS422 levels. Since RS422 was designed for differential signaling, there are pairs of signal wires instead of a single-ended ground that is shared between the Rx and Tx lines. For an RS232 protocol, one can tie the two negative wires of the NMEA 0183 signal pairs together to act as the ground.

The NMEA 0183 defaults to 4800 baud, 8 data bits, no parity, 1 stop bit, no handshake (4800-8-N-1). It is used to exchange way-point and other information between displays, GPS devices, and transponders.

When Raveon’s M7-GX transponder is connected to the Raymarine E80 using the NMEA 0183 connection, the GPS radio transponder can put icons on the screen of the E80. As the transponder receives updated positions from other vehicles, it updates the position of the tracked vehicle icons on the E80 display.

Raymarine E80 Wiring

The Raymarine E80 has the 5-pin NMEA 0183 connector shown to the right, located on the rear of the unit. NMEA 0183 is a common communications format for marine applications. See the following diagram for general wiring connections.

The recommended cable for interfacing to this NMEA 0183 port is the R08004 NMEA cable. It is a 5-wire cable with an Rx pair (differential + and -), Tx pair (differential + and -) and drain wire (unused). For the case of the E80-M7 system, the only used connections will be the white wire (NMEA input +) and the green wire (NMEA input common). These NMEA 0183 cables come in various forms so make sure to verify that the correct wire pairs (Rx, Tx or both) are included in the cable assembly before purchasing. Here is a picture of the full cable assembly with both Rx and Tx wires:

Wiring the Serial Cable

The E80 NMEA 0183 port must be connected to the M7-GX GPS transponder. This connection will allow the M7-GX to put icons on the screen of the E80 display, showing the location of other tracked vehicles. The M7-GX GPS transponder uses a 9-pin DB9 as it’s serial connection. Solder the E80 data cable wires onto a DB9 connector and plug the DB9 into the M7 transponder as shown below:

Connect the white wire (NMEA input +, pin 2) on the R08004 cable to the serial data output (pin 2) on M7-GX DB9 connector. Then, connect the green wire (NMEA input -, pin 1) on the R08004 cable to the ground of the M7-GX DB9 connector. You do not need to connect the brown, yellow or braid wires on the R08004 cable so you can trim them off if desired.

Configuring the E80

Verify that the serial data communications are set to 4800 baud-8-N-1 on the E80.

Configuring the M7 GX Transponder

Raveon has a designed the M7 GX transponder to work with the E80 display or any other NMEA 0183 display that can accept the “$GPWPL” NMEA message. The $GPWPL is an industry standard message that many GPS displays interpret as a waypoint command. The M7 GX outputs this $GPWPL message to put icons on the screen of the E80, and to move the icons around on its screen.

To configure the M7 transponder to output the $GPWPL message, set the M7 GX to GPS mode 4. To do this, put it into the configuration mode by send the +++ into the serial port. The M7 will respond with an OK. Type GPS 4 and press enter to put it into GPS 4 mode. GPS 4 is the mode that causes the M7 GX to output $GPWPL messages whenever it receives a status/position message over the air
.

RF Power Density vs Distance

The RF power density at any distance from a transmitter that is radiating RF power will depend upon the antenna gain, power into the antenna, and the distance from the antenna. The table below illustrates haw fast the RF power power density drops off given certain distances from the radiating antenna and the power into the antenna.

11 meters from a 125 watt transmitter is about the same RF power density as 1 meter from a 2 watt transmitter.

Coax cable attenuation.

All coax cable looses power. The amount of power loss in a coax cable depends upon the length of the cable, the type of cable, and the frequency it is being used at.

The following tables are helpful for predicting the amount of signal loss through a coax cable. For example, if you use a 50 foot section of LMR-400 cable in your data radio modem system that operates a 450MH, you will loose 1.6dB.